Rates of oxygen consumption were determined daily from birth to 19 days of age in lean and obese mice. As early as 5 days after birth obese mice consumed less oxygen than lean mice. Body fat was 38% greater in obese than lean by 7 days after birth. Alterations in energy metabolism occur very early in the life of the obese mouse. Muscle turnover was estimated from the excretion of 3-methylhistidine in the urine. The fractional breakdown rate of muscle averaged 6% per day at 17 weeks of age in both lean and obese mice. However, accumulation of muscle was less in the obese suggesting a decrease in rate of muscle protein synthesis in obese.